Memory provides data storage for electronic systems. Flash memory is one type of memory, and has numerous uses in computers and other devices. For instance, personal computers may have BIOS stored on a flash memory chip. As another example, flash memory is used in solid state drives to replace spinning hard drives. As yet another example, flash memory is used in wireless electronic devices as it enables manufacturers to support new communication protocols as they become standardized, and to provide the ability to remotely upgrade the devices for improved or enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. The flash memory may be erased and reprogrammed in blocks. NAND may be a basic architecture of flash memory. A NAND cell unit comprises at least one selecting device coupled in series to a serial combination of memory cells (with the serial combination commonly being referred to as a NAND string). Example NAND architecture is described in U.S. Pat. No. 7,898,850.
Flash memory cell strings have historically been arranged to extend horizontally, although vertically extending memory cell strings are now being considered. One goal in fabrication of vertical memory cell strings is to reduce the horizontal area of the substrate occupied by the memory cells as compared to horizontally extending memory cell strings, albeit typically at the expense of increased vertical thickness. Nevertheless, vertically orienting memory cell strings can create horizontal packing density considerations not present in horizontally oriented memory cell string layouts.